This invention relates to breathing apparatus and in particular to demand valves for single stage breathing apparatus.
Breathing apparatus with which the invention is used comprises a cylinder containing a compressed breathable gas (usually air) and a face mask. The user carries the cylinder on his back and wears the face mask, the air being supplied from the cylinder to the face mask. Attached to the face mask is a demand valve which passes the compressed air, at a breathable pressure, from the cylinder to the face mask when the wearer of the face mask inhales and "demands" air. Breathing apparatus of this type is termed single stage if the pressure of the compressed air in the cylinder is reduced to a breathable pressure in the demand valve in a single stage. Such demand valves are often called single stage demand valves.
A further kind of demand valve is known as a second stage demand valve or regulator because the pressure of the compressed air in the cylinder is reduced to a substantially constant intermediate pressure, typically 100 to 120 p.s.i., air at this intermediate pressure then being fed to the second stage demand valve which reduces the air pressure to a breathable pressure. However, as noted above the invention is concerned with single stage demand valves.
The invention aims to provide a single stage demand valve which allows an emergency supply of breathing gas to pass through the demand valve should the valve member of the demand valve stick or seize in the closed condition. Provision of such an emergency by-pass is obligatory in some countries, notably the United States.
Prior demand valves have by-pass valves which are single on/off valves having a movable valve member providing metering of the air flow. Air at cylinder pressure enters the by-pass valve, which reduces the air pressure to a suitable flow in dependence upon the degree of opening of the by-pass valve. As time passes, the pressure in the cylinder falls and the emergency air flow to the wearer falls proportionately. Therefore, the wearer must repeatedly adjust the opening of the by-pass valve to obtain the desired air flow.
The invention aims to provide a single stage demand valve which renders this repeated adjustment unnecessary by, in effect, compensating for the falling pressure in the air cylinder.